The objectives of the proposed research are to relate a) the molecular damage produced by ionizing radiation in DNA to the impairment of function of the DNA, and b) the removal of that damage and the subsequent recovery of function to metabolic repair systems dependent upon protein synthesis. Ribosomal RNA (rRNA) synthesis on the extrachromosomal rDNA gene copies in Tetrahymena and on the amplified chromosomal rDNA gene copies in mammalian cells provides a unique model system for defining the coordinated structural and functional modifications of irradiated genetic material and its repair within the eukaryotic cell. We have already determined that rRNA synthesis is inhibited by gamma-radiation and subsequently recovers. The rDNA will be isolated from the irradiated cells during and after the period of inhibition, and strand-breaks, alkali-labile sites, enzyme-sensitive sites, and base damage measured. The kinetics of the repair or removal of each type of damage will be determined and compared to the kinetics of transcription, processing, and transport of rRNA. Chemical sensitizers and protecting agents will be used to identify the types of damage leading to impaired genetic function, and conditions which inhibit enzymatic reactions in general or protein synthesis in particular will be imposed to assess the presence or absence of metabolic repair systems which do or do not depend upon protein synthesis. A detailed understanding of these radiation-induced modifications in transcription and translation should provide a conceptual framework for evaluating the roles of protein synthesis in the induction of and/or recovery from mitotic delay, chromosome aberrations, premutational damage, and sublethal and potentially lethal damage, and for manipulating the radiation sensitivity of normal and cancerous cells in order to enhance or prevent radiation damage and improve the treatment of human tumors.